In the past few years, the price paid for natural gas used as fuel and a chemical feedstock has been steadily increasing. The desire to sell increasing amounts of natural gas has led, to some extent, to the exploration of new and forbidding areas for additional supplies of gas. However, some newer fields, although quite large, contain gas having only 25% to 40% methane. The remaining 60% to 75% is typically a mixture of acid gases, principally carbon dioxide, but also hydrogen sulfide, carbonyl sulfide, carbon disulfide, and various mercaptans.
Carbon dioxide acts as a diluent and, in the amount noted above, lowers the heat content of the natural gas to the point it may not even burn. The sulfur-bearing compounds are at best noxious and may be lethal. In addition, in the presence of water, these components render the gas very corrosive in nature. Clearly, it is desirable to remove acid gases to produce a sweet and concentrated methane gas having a heating value of near 1,000 BTU/SCF either for delivery to a pipeline or conversion to LNG.
Separation of carbon dioxide from methane is not one made with ease. Consequently, significant work has been applied to the development of methane/carbon dioxide separation methods. The processes can be separated into four general classes; absorption by physical solvents, absorption by chemical solvents, adsorption by solids, and distillation.
Currently, cryogenic distillation is considered one of the most promising methods of separating acid gases, particularly carbon dioxide, from methane. The high relative volatility of methane with respect to carbon dioxide makes such processes theoretically very attractive. However, the methane/carbon dioxide distillative separation has a significant disadvantage in that solid carbon dioxide exists in equilibrium with vapor-liquid mixtures of carbon dioxide and methane at particular conditions of temperature, pressure, and composition. Obviously, the formation of solids in a distillation tower has the potential for plugging the tower and its associated equipment. Increasing the operating pressure of the tower will result in warmer operating temperatures and a consequent increase in the solubility of carbon dioxide, thus narrowing the range of conditions at which solid carbon dioxide forms. However, additional increases in pressure will cause the carbon dioxide-methane mixture to reach and surpass its critical conditions. Upon reaching criticality, the vapor and liquid phases of the mixture are indistinguishable from each other and therefore cannot be separated. A single-tower equilibrium separation operating in the vapor-liquid equilibrium region bounded between carbon dioxide freezeout conditions and the carbon dioxide-methane critical pressure line may produce a product methane stream containing 10% or more carbon dioxide. By comparison, specifications for pipeline quality gas typically call for a maximum of 2%-4% carbon dioxide and specifications for an LNG plant typically require less than 100 ppm of carbon dioxide. Clearly, a distillative separation at the above conditions is unacceptable.
Various methods have been devised to avoid the conditions at which carbon dioxide freezes and yet obtain an acceptable separation. Two processes which utilize additives to aid in the separation are disclosed in U.S. Pat. No. 4,149,864 to Eakman et al, issued Apr. 17, 1979, and U.S. Pat. No. 4,318,723 to Holmes et al, issued Mar. 9, 1982.
Eakman et al discloses a process for separating carbon dioxide from methane in a single distillation column. If insufficient hydrogen is present in the column feedstream, hydrogen is added to provide a concentration from about 6 to 34 mole percent, preferably from about 20 to about 30 mole percent. The separation is said to take place without the formation of solid carbon dioxide. The tower pressure is preferably held between 1025 and 1070 psia.
Holmes et al adds alkanes having a molecular weight higher than methane, preferably butane, to the tower feed to increase the solubility of carbon dioxide and decrease its freezing temperature line. The additive n-butane is added at an amount from about 5 moles to 30 moles per 100 moles of feed.
Neither of these processes suggest the use of helium as an additive to a stream containing carbon dioxide and methane to raise the critical pressure of the mixture and therefore allow effective distillation of that mixture without carbon dioxide freezeout.